Cold cathode gas tube amplifier



f April 1, 1952 Filed May 2, 1951 H. JACOBS ETAL 2,590,863

cow CATHODE GAS TUBE AMPLIFIER 2 SHEETS-SHEET l INVENTORS- HAROLD JACOBS JACK R. MARTIN l atented Apr. l, 1952 COLD CATHODE GAS 'TUBE AMPLIFIER Harold Jacobs and Jack R. Martin, Long Branch, N. J assignors to the United States of America as represented by the Secretary of the Army Application May 2, 1951, Serial No. 224,244

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 6 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

The present invention relates to gaseous discharge devices. More particularly, the present invention relates to multi-electrode glow discharge devices capable of providing continuously controlled linear amplification.

It is known that gaseous discharge devices such as the conventional thyratron and cold cathode triode can provide a relay type of control action. Such devices, however, are incapable of continuous linear control, which is necessary to the undistorted transfer and amplification of signal energy.

Consequently, the function of signal amplification has been relegated to vacuum discharge devices wherein continuous control is obtainable. In the utilization of vacuum discharge devices it is necessary to provide a copious supply of electrons through the mechanism of supplying considerable power for cathode or filament heating, and in portable compact equipments the source of heating power frequently becomes a limiting factor as to the ultimate minimum bulk of the equipment.

Cold cathode discharge devices, such as the grid controlled cold cathode discharge device is based on the principle of utilizing for the electron source a glow discharge produced between two electrodes. An electric field is developed between one of the glow discharge electrodes and an amplifier anode to produce an electron flow between the discharge and the anode. Potential variations of the electric field are then produced by means of a grid disposed between the anode and the glow. Such a device, which obviates the necessity for a source of heating power, requires a plurality of sources of potential to provide the functions of sustaining the discharge and developing the electric field.

. Electron discharge devices have in the past, generally, been considered as high input impedance devices. When coupling a low impedance' source to the input circuit of such a discharge device, it is necessary to utilize an impedance matching device, such as a transformer, with the consequent frequency limitations, inherent distortion, and expense.

Accordingly, an object of the present invention is to provide an improved cold cathode gaseous discharge device capable of uninterrupted linear control.

1 Another object of the present invention is to provide an improved cold cathode gaseous discharge device having a relatively low input impedance.

A further object of the present invention is to provide an improved cold cathode gaseous discharge device capable of providing linear amplification and requiring a minimum of operating power.

According to the present invention there is provided a gaseous discharge device comprising a pair of emitter electrodes in coextensive juxtaposition and having adjacent surfaces consisting of material possessing a low work function.

There is also provided a filamentary collector electrode disposed in proximity to and in symmetry with the emitter electrodes. The electrodes are maintained in an atmosphere of noble gas at a predetermined critical pressure. A polarizing potential is applied between one of the pair of emitter electrodes and the collector electrode and a load impedance is connected between the emitter electrodes. Signals then, applied to the polarizing circuit are amplified and developed across the load resistor.

A better understanding of the present invention may be had by reading the following description in connection with the accompanying drawings in which:

Fig. 1 is a view in perspective of a gaseous discharge device constructed in accordance with the present invention;

Fig. 2 is a view partly in cross section of a portion of the device shown in Fig. l and illustrating a further embodiment of the present invention;

Fig. 3 is a plan view of the device shown in Fig. 2 illustrating one embodiment of collector electrode structure as provided by the present invention;

Fig. 4 is a schematic circuit diagram illustrating a circuit application of the device provided by the present invention; and,

Figs. 5, 6 and '7 are graphs showing curves illustrating respectively the relationship between voltage gain and load resistance when a tube as provided by the present invention is filled with different gases and utilized in a circuit as illus trated in Fig. 4.

Referring now to the drawings, and in particular to Figure 1, there is shown a gaseous discharge device having a sealed envelope IU of glass or other suitable material. Three inleads [2, I4 and Hi are sealed into one end of the envelope through a conventional seal to allow electrical connection to the electrodes within the envelope l0. Other inleads illustrated in the drawing are utilized for mechanical support and for activation of the tube electrodes. As these other inleads do not enter into the electrical functioning of the device it is believed that further discussion relating to them is unnecessary.

A pair of electron emitter electrodes or cathodes l and 20 are supported in coextensive juxtaposition by appropriate structure. The adjacent surfaces 22 and 24 of the cathodes l8 and 24 are composed of a material having a low work function. Such a material may be barium oxide, barium strontium oxide or other materials from which electrons may be freely liberated.

It is noted at this time that the low work function material must be only on the adjacent cathode surfaces 22 and 24 as clearly illustrated in Fig. 2 of the drawing. The restricting of the coating material to the adjacent surfaces has been found to be necessary to cause the glow discharge to appear between the cathodes l8 and 20. If the coating material is allowed to be present on the outer periphery of the cathodes the glow will be diffused and the operating characteristics of the device will be materially altered.

Within the envelope l0 and in the plane defined by the gap formed by the cathodes l8 and 20 there is an electron collector electrode or anode 26, which may be, as illustrated in Fig. 1, in the form of a hooked probe adjacent one side of the gap. It is, of course, to be understood that the supporting structure and the physical forms of the electrodes shown in Fig. 1 of the drawings are for the purpose of illustration only and that other modifications can be utilized without departing from the scope of the invention.

An example of an embodiment of the present invention which has been successfully used is illustrated in Figs. 2 and 3 of the drawings. In this instance, the anode 26 is in the form of a wire loop having an inner diameter larger than the outer diameter of the cathodes by an amount equal to twice the desired spacing between the gap and the anode.

A gas tube, as illustrated in Figs. 1, 2 and 3, is illustrated as being connected in an electrical circuit in Fig. 4, wherein a source of direct current energizing potential or battery 28 and a current limiting resistor 30 are connected in series between the cathode 20 and the anode 26. The circuit for the gas tube is completed by a load resistor 32 connected between the two cathodes l8 and 20. Input signals are applied across the current limiting resistor 30 at the terminals 35 and appear in undistorted amplified form across the load resistor 32 at the output terminals 30.

Before more fully describing the internal operation of the tube provided by the present invention, it is believed necessary to briefly discuss certain physical laws of gas discharge. The first is known as Paschens law and states that in a gas the sparking potential is a function of the gas pressure and the spacing of the electrodes and that there is a minimum potential below which sparking will not take place. An'examination of this law would lead one to believe that discharge would take place between the nearest portions of the electrodes.

However, it has been found that Paschens law can be modified by providing electrodes of low work function material. In this instance the same potential will produce a discharge at a 4 greater pressure and distance. This phenorn non is utilized in the present invention.

A second factor which must be considered is the mechanism of voltage regulation in a gas tube. It is generally accepted that the cathode glow region in a gas tube is a space charge limited region of positive ions. As the current through the tube is increased the glow changes in size, but the voltage drop from the glow to the cathode changes only slightly and the current density changes only slightly, provided the surface is uniform in chemical structure and in temperature. The voltage drop across the tube is almost the same as the voltage drop from the glow discharge to the cathode.

Another fundamental mechanism utilized by the present invention is known as the second Townsend coefficient (6), which substantially is a factor indicating the relative number of electrons liberated from a surface per positive ion impinging on that surface. An analysis of this mechanism will indicate that the electron liberation is related inversely to the work function of the electrode surface material. That is, the lower the work function of the material, the more copious the electron yield under a given set of conditions.

It has been found that in an atmosphere of argon, a nickel surface exhibits a second Townsend coefficient in the order of 0.001; whereas, a barium oxide surface in the same atmosphere exhibits a second Townsend coefficient in the order of 0.5. It is, therefore, believed to be obvious that electrodes of nickel, which are commonly used in cold cathode tubes, will provide relatively few electrons as compared to electrodes having a barium oxide coating and under the same conditions.

It has been found that when cathodes having a low work function material on one surface only are disposed so as to form a gap in the order of about 0.001 to .005 inch or less, the closer spacing being better, and an anode is positioned in the plane of the gap at a distance in the order of 0.1 cm. from the periphery of the gap and the electrodes are placed in an atmosphere of noble gas at a predetermined critical pressure, a polarizing potential applied as illustrated in Fig. 4 of the drawing will produce a glow discharge which is confined to the gap between the cathodes. If the low work function material is on cathode surfaces other than that immediately adjacent the other cathode, the glow discharge becomes diifused and the effectiveness of the tube is substantially reduced. The gas pressure also must be carefully selected. If the gas pressure is too high the tube will arc, and if the gap pressure is too low the glow will be too diffused. It is believed. therefore, to be clear that for optimum operation the cathode material, the cathode spacing and the gas pressure must be carefully selected.

Further, there appears to be an optimum load resistance in ohms of the circuit illustrated in Fig. 4. The results shown in Fig. 5 were obtained with a tube as provided by the present invention filled with argon at a pressure of 20 mm. of mercury and the variable was the current limiting resistor 30. Curve A represents the results with a resistance of 10,000 ohms, curve B represents the results with a resistance of 2200 ohms and curve C represents the results with a resistance of 1500 ohms. It is believed to be clear from these curves that the tube is a low input impedance device and that the results are substantially independent of the input circuit.

In Fig. 6 of the drawings, the ordinate and abscissa of the graph are respectively gain and load resistance of the circuit illustrated in Fig. 4. However, in this instance the tube was filled with krypton gas and the variable was the gas pressure. Curve D represents the results with a pressure of 8 mm., curve E represents the results with a pressure of 14 mm., curve F represents the results with a pressure of 20 mm. and curve G represents the results with a pressure of 32 mm. It is believed to be clear from these curves that there is a definite optimum and that the optimum load resistance is, at least in part, a function of gas pressure.

The curves shown in Fig. 7 of the drawings represent the relationship, in a neon filled tube, between the load resistance and the gas pressure. Curve l-I represents the results with a load resistance of 100,000 ohms, curve I represents the results with a load resistance of 60,000 ohms and curve J represents the results with a load resistance of 1 megohm. It appears from an examination of the curves that a critical pressure for optimum gain exists in the region of 45 mm. of mercury and that the ultimate gain is related to the magnitude of the load resistance.

It has been found that when voltage is applied, as by battery 28 in Fig. 4, that the glow discharge must be between the cathodes l8 and 20. Since the glow discharge appears in the gap between the surfaces of the low work function material there is a coupling between the two cathodes l8 and 20. An electron flow is established from the cathode 20 which is sufficient, if the voltage of the battery 28 is in the order of one hundred volts, to develop a voltage drop across the load resistance of about 40 volts.

This voltage drop is in effect a bias voltage existing between the cathodes l8 and 20. When a signal voltage is applied across the current limiting resistor 30 the variation in current produced thereby produces a corresponding variation in current in the load resistor 32, which, in turn, causes a variation in the voltage drop across the load resistor 32 which represents an amplified reproduction of the input signal. Batteries'could be used with a smaller output resistor to obtain gain and still have smaller time constant and lower resistance coupling.

It has been found that cathodes of nickel having a high work function did not provide sufficient coupling between the cathodes l8 and 20. 'It has also been found that hot cathode tubes were-|far inferior in operation to the cold cathode structure above described. Experimental results indicated that when cathode 20 was operated at room temperature best results were obtained. As the cathode was heated, the gain grew progressively worse.

It is believed that a probable explanation of the mechanism of this tube resides in the effect of positive ion bombardment of the cathode 20 due to the glow discharge established and maintained by the cathode I8 and the anode 26. However, it is to be understood that some effect may be produced by metastable ions and the photoelectric effect.

Experiments which have been conducted with tubes constructed in accordance with the present invention indicate that static gains from 100 to 1000 are possible and even with imperfect experimental structures gains of can be consistently achieved. The achieving of the higher gain resides in providing closer spacing and operating the first cathode l8 as a normal cold cathode application as a direct current or alternating current voltage amplifier or power amplifier with very long life as glow tubes are known to have a life in the order of 10,000 hours.

What is claimed is:

1. A gaseous discharge device comprising a sealed envelope containing an atmosphere of noble gas at a predetermined critical pressure and enclosing a pair of electron emitter electrodes each having a surface of low work function material, said electrodes being disposed with said surfaces in close juxtaposition to form a gap in which a discharge may be maintained, and a collector electrode disposed external to said emitter electrodes in the plane of the gap formed by said collector electrodes.

2. A gaesous discharge device comprising a sealed envelope containing an atmosphere of noble gas at a predetermined critical pressure and enclosing a pair of electron emitter electrodes each includin a plane surface, said plane surface having a coating of material characterized by a low work function, said electrodes being disposed with said surfaces in close juxtaposition to form a gap in which a discharge may be maintained, and a collector electrode disposed external to said emitter electrodes and in proximity to said gap.

3. A gaseous discharge device comprising a sealed envelope containing an atmosphere of noble gas at a predetermined critical pressure and enclosing a pair of electron emitters each having one surface of low work function material, said electrodes being disposed with said low work function material surfaces in close juxtaposition to form a gap in which a discharge may be maintained, and a collector electrode disposed symmetrically with said emitter electrodes and in proximity to said gap. I

4. A gaseous discharge device comprising a sealed envelope containing an atmosphere of noble gas at a predetermined critical pressure and enclosing a pair of cylindrical cathodes each having a plane end surface of low work function material and peripheral surfaces of high work function material, said cathodes being disposed in coextensive juxtaposition, said end surfaces being adjacent, and a filamentary anode disposed external to and symmetrical with respect to said cathode.

5. A gaseous discharge device comprising a sealed envelope containing an atmosphere of noble gas at a predetermined critical pressure and enclosing a pair of cylindrical cathodes disposed in coextensive juxtaposition to form a gap in which a discharge may be obtained, the adjacent' surfaces of said cathodes consisting of a material having a low work function, and a ring anode having a diameter larger than said cathodes and disposed concentrically with said oath- The electrodes are odes in the plane of said gap, whereby electronsv may be caused to flow from the discharge'to said anode.

6. A gas tube amplifier circuit, comprising in combination, a gaseous discharge device com-. prising a sealed envelope containing aniatmosphere' of noble gas at a predetermined critical pressure and enclosing a pair of electron emitter electrodes each having a surface of. low. work,

function material, said electrodes being disposed with said surfaces in close juxtapositionito form.

a gap in which a discharge may be maintained,

HAROLD JACOBS. JACK R. MARTIN.

No references cited; 

